Souring milk would actually be a chemical change because it changed the properties alltogether. You can't change soured milk into it's original taste.
As the name implies, anaerobic means there is no presence of oxygen. So if it will take place, it will only take place in little or no oxygen which is in the last sentence in the following choices. First and second choice are wrong for there is a need of oxygen required when there is no oxygen is needed and the presence of nitrogen in water.
I think its B, however not 100% sure. Hope that helped, sort of
